Conventionally, silicon is selectively etched using a mixture of acids comprising fluoric acid, nitric acid, and acetic acid. Dash has developed a method for evaluating defects in silicon using an etchant made up of a 1:3:12 volumetric ratio of these acids.
A later etching method has been proposed using an etchant with a volumetric ratio of these acids of 1:3:8.
Because the etching rate of this etching method depends on the concentration of impurities in the crystals, the etchant has been attracting attention as an isotropic selective etchant.
However, there is the problem that the etching characteristics of this selective etchant have a tendency to change, causing poor controllability. Accordingly, Muraoka et al. have proposed a method for controlling selectivity by the addition of a hydrogen peroxide solution into tile etchant (Japanese Laid Open Patent Application 49-45035).
An example of an etching device using the method of Muraoka et al. is described in the following two papers:
"CONTROLLED PREFERENTIAL ETCHING TECHNOLOGY", H. Muraoka, T. Ohhashi, and Y. Sumitomo, Semiconductor Silicon 1973, the Electrochem. Soc., Soft-Bound Symposium, Sir Princeton N.J., H. R. Huff and R. R. Burgess, eds., Electrochem. Soc.; pp 327 to 338, 1973". PA1 a semiconductor electrode immersed in the processing liquid; PA1 an opposing electrode which is also immersed in the processing liquid, opposing said semiconductor electrode; PA1 means for detecting an electric potential difference between said semiconductor electrode and said opposing electrode; and PA1 control means for uniformly controlling the nitrite ion concentration in the processing liquid from the electric potential difference between said semiconductor electrode and said opposing electrode, detected by said detection means. PA1 a semiconductor electrode immersed in the processing liquid; PA1 an opposing electrode which is also immersed in the processing liquid, opposing said semiconductor electrode; PA1 means for detecting an electric potential difference between said semiconductor electrode and said opposing electrode; and PA1 chemical addition means for adding nitrite ions or a chemical which generates nitrite ions to the processing liquid corresponding to the electric potential difference between said semiconductor electrode and said opposing electrode, detected by the detection means. PA1 a semiconductor electrode to which a voltage is applied, immersed in the processing liquid; PA1 an opposing electrode which is also immersed in the processing liquid, opposing said semiconductor electrode; PA1 electric potential difference detection means for detecting an electric potential difference between said semiconductor electrode and said opposing electrode; and PA1 voltage control means for controlling the voltage applied to said semiconductor electrode, corresponding to the electric potential difference between said semiconductor electrode and said opposing electrode, detected by said electric potential difference detection means. PA1 absorption detection means for detecting the absorption of the processing liquid; and PA1 control means for uniformly controlling the nitrite ion concentration in the processing liquid corresponding to the absorption of the processing liquid detected by said absorption detection means. PA1 in the etching device described above, said absorption detection means comprises a spectrophotometer. PA1 absorption detection means for detecting the absorption of the processing liquid; and PA1 chemical addition means for adding nitrite ions or a chemical which generates nitrite ions to the processing liquid corresponding to the absorption of the processing liquid detected by said absorption detection means.
In this device, an etchant made up of fluoric acid, nitric acid, and acetic acid (volumetric ratio 1:3:8) is filled into an etching tank and continuously agitated using a stirring bar, while an object of the etching process, such as a silicon substrate or the like held by a jig, is etched.
In this device, the concentration of nitrite ions formed as the result of a dissolution reaction with the etchant is monitored using an electron meter by measuring the electric potential between a pair of electrodes, specifically a platinum electrode and a carbon electrode, immersed in the etchant.
The concentration of the above-mentioned nitrite ions must be uniformly controlled during the etching process from start to finish so that the etching characteristics of the etchant do not change. In this method, a reagent, for example, a hydrogen peroxide solution, is added to oxidize the nitrite ions so that the potential between the electrodes is kept uniform.
The inventors of the present invention have also proposed a method of etching process in which nitrite acid or a chemical which produces nitrite ions is added to the processing liquid. (Japanese Patent Application 3-261948 and Summary of Lectures of the 1991 Electrochemical Autumn Conference (the site of the meeting is Nagoya in Japan), No. 2H25, Page 174, "Micromachining of Silicon by means of Isotropic Selective Etching."
However, in this conventional method of etching process, a metal electrode such as platinum or gold and a carbon electrode are directly immersed in the etchant. Therefore, when monitoring the etchant using such conventional methods, the monitor electrode, which is the platinum electrode or the gold electrode, is dissolved by the etchant and is mixed with the etchant, and this leads to the danger that the material to be etched will be contaminated by the metal.
This type of metal contamination is a problem because the life expectancy of the silicon is reduced, or the like. Therefore, there is the problem that the adjustability of the semiconductor fabricating process, which requires a high degree of cleanliness, is not good.
For example, when a platinum electrode and a gold electrode are used, about 4 ppb of gold are detected when the used etchant is analyzed in a graphite furnace analysis method.
In addition, a uniform nitrite ion concentration is maintained in the conventional method by adding a hydrogen peroxide solution to the etchant, and the characteristic of the electric potential difference between monitor electrodes and the characteristic of the ion concentration of the etchant are affected in both regions, a plateau region (or a flat region) and a jump region.
The hydrogen peroxide solution is added in this plateau region, and even when the ion concentration is reduced no change is observed in the electric potential difference, therefore the etchant is not monitored in this plateau region.
Accordingly, the monitoring must be set so that this detection is made in the jump region.
For example, when the platinum and the gold electrodes are used, if the concentration of the nitrite ions slightly exceeds 100 ppm (converted to nitrite ions, including chemicals formed from the nitrite ions by means of a reaction in equilibrium) the process has already entered the plateau region and the electric potential between the electrodes still does not change for a change in the nitrite ion concentration. For this reason, it is not possible to estimate the nitrite ion concentration in the plateau region, and there is no process monitor function.
Because metal electrodes and the like are immersed in the etchant when this type of conventional monitoring process is used, the danger of fouling is high and it becomes impossible to measure the correct nitrite ion concentration
In addition, even using this method based on the conventional etching device, in tests dealing with three-dimensional micro-machining of an intelligent sensor representative of a manufacturing process for an acceleration sensor, the etching reaction did not proceed to completion, so specifically, there was the problem that the etching did not proceed to completion.
For example, a conventional method, in which a hydrogen peroxide solution is added to an etchant to maintain a uniform nitrite ion concentration in the liquid, cannot be applied in the case where the etching reaction is out of control.
In general, the nitrite ion concentration is increased after starting the etching process. But, in a micro-machining representative intelligent sensor process, the nitrite ions are dispersed and diffused in the etchant, so that the nitrite ion concentration is seen to decrease.